It is known that a body of water such as a tank, pond, pool, lake, bay, canal or marina may require the addition of oxygen, or other gas for the raising of fish, shellfish, algae, or other cultured specie. The water body may also require aeration for the bacterial digestion of waste as in sewage treatment, or in order to raise the amount of oxygen near the bottom to water quality standards as set by regulatory agencies. A body of water may also require aeration if it has become either eutrophic or stagnant, or if it contains a high level of unwanted or toxic gas. It is therefore often necessary to aerate, agitate, or circulate water in water impoundments to obtain the desired gas content, water quality, mixing action, or destratification.
Many techniques already exist for oxygenating, aerating, gas stripping, and mixing water and other liquids. Some methods use large motor-driven propellers for bringing about oxygenation, but these are quite expensive to purchase and to maintain. Gas diffusion or sparging is another method which is well known in the fields of aquaculture, wastewater treatment, and pond care. However, these prior attempts proved to be particularly expensive in large, shallow bodies of water. Costs rose with the large expenditure of energy required per unit area, logistical complexity, maintenance requirements, multiple electric motors and associated wiring, and the high air pressures required.
A further increase in the expense of harvesting the aquaculture crop is encountered if traditional aeration devices have been used. In harvesting, a large net must be drawn through the pond, requiring the prior removal of the aeration device(s). In addition, the operation of conventional aeration devices residing on the bottom of the pond may cause the formation of craters or indentations in the pond bottom that would enable fish to escape capture by the net. Because of factors such as these, and because the profit margin in aquaculture is small, it has not been possible before the present invention to evolve a cost effective method of aerating and maintaining large aquaculture ponds, such as the shallow ponds in which catfish and shrimp are grown, as will enable higher stocking densities and reduced mortalities, and obviate the need for mechanical aeration on an emergency basis.
It is a known fact that circulation can be induced in a deep body of water, by the use of submerged air diffusers, propellers, or other devices which create mixing currents that cover a broad area, creating a large "sphere of influence" leading to good aeration. However, the aforementioned air diffusers used in shallow bodies of water produce comparatively small spheres of influence, for equivalent energy inputs. This situation results from the increased resistance to horizontal flow as the water becomes shallower. In shallow water many more devices are required per unit of surface area in order to create the mixing currents, essential to successful catfish farming.
It has been shown experimentally (Parker, U.S. Fish and Wildlife) that 16,000 pounds of catfish can be raised per surface acre of water with the use of continuous aeration. This, however, cannot be justified commercially because the cost of aeration is greater than the increased profits realized from the sale of the additional fish. Some aeration devices which have been tried in large ponds are typical of those used in the waste water treatment industry and show little promise to the aquaculturist because of the vastly different conditions-- namely, the need for vertical circulation over a large shallow area. Very little continuous aeration is currently utilized by catfish farmers except on an experimental basis. Emergency aeration, however, is used to prevent fish kills when such kills appear to be imminent. The commonly accepted maximum figure for the number of pounds of catfish able to be grown without continuous aeration (but with emergency aeration available) is approximately 5,000 pounds per acre.
As will be seen hereinafter, it is the purpose of this invention to provide aeration at such a low cost as to enable 9,000 pounds or more of catfish to be grown per acre, with an energy input of only 300 Watts per acre. Catfish farmers might thereby realize far greater profits than by utilizing the teachings of the prior art. Catfish are used here only as an example, and there is an equally dramatic increase with other species.
Many catfish farmers have gone to the expense of creating aeration arrangements involving numerous lengthy submerged pipes or tubes that serve to deliver air under pressure to an array of diffuser devices residing on the bottom of the pond, or other body of water. However, the shallow diffusers and the diffusers near the air source tend to deliver a disproportionately large amount of the supplied air to a comparatively small segment of the pond, while remotely located diffusers are starved for air. Consequently remote parts of the pond are not aerated properly.
Fish farmers have tried in the past to cope with this problem by several different techniques, including the use of dropping orifices, valves, or the like in an attempt to apportion the air properly. However, this increases expense and complexity, and raises significantly the pressure level needed at the air pump in order to assure that each submerged diffuser receives an appropriate amount of air. Other efforts to apportion air properly have included:
1. The use of enough submerged pipes or tubes so that each diffuser, or each small group of diffusers, will have a private air supply; PA0 2. Perfect leveling of all diffuser devices with air manifolds of inordinately large diameter to minimize pressure drop between diffusers; and PA0 3. Having high resistance to air pressure in each diffuser or orifice (as used by Hinde Company, Highland Park, Ill.) thereby minimizing the pressure differences between diffusers fed from a common supply line.
However, not only are these expensive, but also it is a great hinderance at the time the fish are to be harvested, because of the substantial obstruction presented to the use of nets.
Another important disadvantage of the submerged system is its dependence upon the pond having a relatively uniform bottom contour. The depth at which certain diffusers reside, if different from the depth of the other diffusers, brings about the need for the use of line restrictions such as dropping orifices or the like, which not only increase expense, and are prone to clog, but also more importantly, they force the aquaculturist to use the high pressure system, with its attendant and distinct disadvantages.
It was in an effort to obviate these and other severe problems that I developed the novel low pressure aeration system described hereinafter that has proved exceedingly cost effective.